Process for the introduction of carboxyl groups into aromatic hydrocarbons



United rates Patent PROCESS FOR THE INTRODUCTION OF CAR- GROUPS INTO AROMATIC HYDROCAR- Bruno Blaser, Dusseldorf-Urdenbach, Hubert Schirp, Dusseldorf, and Werner Stein, Dusseldorf-Holthausen, Germany, assignors to Henkel & Cie. G.m.b.H., Dusseldorf-Holthausen, Germany, a corporation of Germany No Drawing. Filed Sept. 25, 1957, Ser. No. 686,007

Claims priority, application Germany Oct. 2, 1956 12 Claims. (Cl. 260--S15) This invention relates to the introduction of carboxyl groups into aromatic hydrocarbons to produce aromatic hydrocarbon carboxylic acids in the form of their salts. The invention more particularly relates to the carboxylation of aromatic hydrocarbons by direct and selective introduction of carbon dioxide to produce polycarboxylic acids of a symmetrical nature such as terephthalic acid and trimesic acid.

This selective carboxylation is accomplished by heating the aromatic hydrocarbon starting material in the presence of an acid-binding agent, and a source of carbon dioxide under anhydrous conditions. For example, terephthalic acid and trimesic acid may be produced from benzene in such a manner.

It is an object of this invention to produce aromatic hydrocarbon carboxylic acids and their derivatives by selective carboxylation of aromatic hydrocarbons.

Another object of this invention is to produce terephihalic acid by selective carboxylation of benzene.

A further object of this invention is to produce naphthalene-2,6-dicarboxylic acid by selective carboxylation of naphthalene.

These and other objects of this invention will become :apparent as the description thereof proceeds.

It is known that aromatic monocarboxylic acids may be produced by heating aromatic hydrocarbons and carbon dioxide in the presence of aluminum chloride. This process requires the use of relatively large quantities of aluminum chloride. The yields obtained thereby are meager.

We have now found that carboxyl groups may be in- :troduced into aromatic hydrocarbons, which may also be substituted with alkyl or cycloalkyl radicals, by heat- .ing hydrocarbons in the presence of carbon dioxide and in the presence of acid-binding agents, and also advantageously in the presence of materials which are capable of binding or reacting with the water formed during the reaction, to temperatures above 350 C. In accordance with this process, salts of aromatic carboxylic acids are obtained in many cases, especially salts of industrially valuable dicarboxylic acids, such as terephthalic acid or naphthalene-2,6-dicarboxylic acid. These salts may be transformed in accordance with known methods into the free acids or their derivatives. In some cases, second- ;ary reaction products such as ketones, for example, are .formed instead of or in addition to the carboxylic acid salts. When substituted aromatic hydrocarbons are used :as the starting materials, the reaction products may someztimes also be salts of unsubstituted carboxylic acids.

Suitable starting materials for the process according to :the present invention are aromatic hydrocarbons, especially benzene, but also toluene, xylene, cumene and :diisopropyl benzene and other benzenes substituted with saturated or unsaturated alkyl or cycloalkyl radicals, and :furthermore naphthalene, diphenyl, diphenylmethane and Patented Aug. 9, 1960 other aromatic compounds which may also be substituted with hydrocarbon radicals, may be employed.

It is preferred to use the carbonates of alkali metals, especially potassium carbonate, as the acid-binding agent. Acid-binding agents are those chemical compounds capable of combining to neutralize the acids produced by the reaction. In place of the carbonates, the salts of other Weak acids may be used; for example, the bicarbonates, formates or oxalates. Similarly, the corresponding compounds of other metals are suitable for this purpose; for example, the carbonates of the alkali earth metals. When using alkali earth metal compounds, other reaction products are often formed, especially different isomers than those produced when alkali metal salts are used. These acid-binding agents may be added as such or formed in situ during the reaction.

The starting materials are preferably heated in the presence of those compounds which are capable of binding the water formed by the reaction, or are capable of reacting with the Water Without interfering with the reaction proper. Such compounds are, for example, the carbides of various metals, such as aluminum carbide, or the carbides of alkali earth metals or alkali metals, such as calcium carbide. Similarly, other compounds of the above-named metals, for example their nitrides and phosphides are suitable for this purpose. Free metals which are capable of reacting with the water at the prevailing temperatures, such as aluminum, may also be used. The binding of the water formed by the reaction may also be accomplished by other methods; for example, With the aid of alkali metal carbonates, especially potassium carbonate, which in this case must be present in excess over that amount which is required for the neutralization of the carboxylic acid formed by the reaction. In accordance with'a further embodiment of the present process, the reaction is simultaneously carried out with carbon dioxide at a pressure such that the 'bicarbonate formed thereby will not decompose at the prevailing reaction temperature.

The above-named starting materials are heated in the presence of carbon dioxide, which may also be present in combined form; for example, in the form of a carbonate. They are preferably heated in the presence of gaseous carbon dioxide under pressure. There is no upper limit to this pressure-that is, the upper limit is set only by the pressure resistance of the apparatus, pumps, etc. which are available. The reaction may, however, also be carried out at atmospheric pressure. In both cases, the process may also be carried out by passing a mixture of benzene vapor and carbon dioxide over a heated mixture of potassium carbonate and aluminum carbide, or by passing a mixture of benzene vapor and carbon dioxide through a fluidized bed of potassium carbonate and aluminum carbide. In place of carbon dioxide, gas

mixtures may be used which contain inert gases, such as nitrogen, methane or argon, in addition to carbon dioxide. It is advantageous to avoid the presence of large quantities of oxygen.

The quantitative ratio of carbon dioxide and aromatic hydrocarbon may vary Within wide limits. Ordinarily an products.

If the above-described starting materials are. solids, it is preferred touse them in a dry :and finely divided form, and inintimate admixture with each other. "In

order to achieve as favorable a reaction as possible among the starting materials, it is preferred to maintain the reaction mixture in motion by stirring or by agitating the reaction vessel. It is further advantageous to admix the solid reaction components with inert solid diluent additives which have a large outer surface or to apply the solid starting compounds to materials with a large outer surface. Suitable additives for this purpose are, for example, asbestos, pumice stone, mineral wool, glass wool, finely divided silicic acid or finely divided aluminum oxide, kieselguhr, or inert salts such as sodium sulfate and the like. It has further been found that the reaction according to the present invention is favorably influenced by certain catalysts. Suitable catalysts are especially the heavy metals, such as zinc, cadmium, mercury, iron, lead, and the like, as well as compounds of these metals, such as their oxides or their salts with inorganic or organic acids.

The separation of the reaction mixture is very simple, as a rule. For example, in the production of terephthalic acid from benzene, the solid components of the reaction mixture may be separated from excess benzene and subsequently dissolved in water. After filtering the aqueous solution to remove insoluble components, the terephthalic acid may be precipitated in accordance with known methods by acidification with inorganic or organic acids. The unreacted aromatic hydrocarbons may, as a rule, be recovered virtually quantitatively, so that the process according to the present invention generally produces practically no side products and produces very good yields. In carrying out the process according to the invention on an industrial scale, the hydrocarbon serving as the starting material may be recycled into the process. The same is applicable to the carbon dioxide used as one of the reaction components, which may again be used for a subsequent run after a suitable purification treatment, if desired. Similarly, the other components of the reaction mixture, for example the catalyst or the inert additives having a large outer surface, may be repeatedly used.

The following examples will enable persons skilled in the art to better understand and practice the invention and are not intended to limit the invention.

Example I 13.8 gm. anhydrous potassium carbonate, 20.0 gm. asbestos (Merck for Gooch filter), 5.0 gm. aluminum carbide (Riedel-De Haen) and 3.0 gm. cadmium fluoride were milled in a ball mill and intimately admixed with each other. The resulting mixture was loosely placed into an autoclave having a net volume of 250 cc., so that the autoclave was about half full. Subsequently, 140 cc. benzene were added, which had previously been dried over sodium, and 50 atmospheres carbon dioxide were introduced until saturation. Thereafter, the autoclave was first heated for 2 hours at 400 C. and then for 4 hours at 430 C. The temperature was measured with a thermometer extending into the center of the autoclave. Since the source of heat was on the outside, the wall temperature was probably somewhat higher than the temperature measured in the interior of the autoclave. The internal pressure was maintained at about 1250 atmospheres. For this purpose about 430atmospheres had to be bled oif during the heating step.

After cooling and releasing the pressure in the autoclave, the solid reaction products were filtered off from excess benzene, dried at 100 C. and extracted with boiling water. The residue, consisting of asbestos, catalyst, and aluminum oxide or hydroxide, was filtered oif. Subsequently, the clear, aqueous solution was heated to the boiling point and acidified with hydrochloric acid. The precipitated terephthalic acid was separated from the solution, extracted with boiling water, filtered and dried. The yield was 3.9 gm., about 24% of the theoretical based on the limiting potassium carbonate.

Example 11 13.8 gm. anhydrous potassium carbonate, 5.0 gm. of the double salt K CdF Cl 5.0 gm. aluminum carbide and 20 gm. asbestos were admixed as described in Example I and placed into an autoclave having a net volume of 250 cc. Thereafter, cc. dry benzene were added and 50 atmospheres carbon dioxide were introduced into the autoclave until saturation. The autoclave was first heated to 400 C. for 2 /2 hours and then at 450 C. for 2 additional hours. The maximum pressure was maintained at 900 atmospheres. The reaction mixture was worked up as described in Example I and yielded 1.7 gm. terephthalic acid. By extraction of the mother liquor with ether 0.3 gm. of a carboxylic acid mixture was obtained which had an acid number of 700.

Example III 13.8 gm. anhydrous potassium carbonate, 3.0 gm. cadmium fluoride and 5.0 gm. aluminum carbide were admixed as described in the preceding examples, and the mixture was placed into an autoclave having a net volume of 250 cc. Thereafter, cc. benzene were added and 50 atmospheres carbon dioxide were introduced into the autoclave until saturation. The autoclave was first heated at 400 C. for 2 hours and thereafter heated at 430 C. for an additional 4 hours. The maximum pressure was maintained at 1250 atmospheres. The reaction mixture was worked up in the above-described manner. The yield of terephthalic acid was 2.4 gm.

Example IV 13.8 gm. anhydrous potassium carbonate, 5.0 gm. aluminum carbide, 3.0 gm. cadmium fluoride and 20 gm. asbestos were intimately admixed with each other as described in Example I, and the mixture was placed into an autoclave having a net volume of 250 cc. Thereafter, 50 cc. dry benzene were added and 50 atmospheres carbon dioxide were introduced until saturation. The autoclave was heated at 430 C. for 4 hours. The maximum pressure was maintained at 375 atmospheres. The reaction mixture was worked up in the manner described in the preceding examples. The yield of terephthalic acid was 1.0 gm.

Example V 27.6 gm. potassium carbonate, 5.0 gm. zinc chloride, 10.0 gm. powdered aluminum carbide and 20 gm. asbestos were milled and intimately admixed in a ball mill. The resulting mixture was loosely placed into an autoclave having a net volume of 600 cc. Thereafter, 250 cc. benzene, which had previously been dried over sodium, were added to the mixture and 50 atmospheres carbon dioxide where introduced into the autoclave until saturation. The contents of the autoclave were then heated for 12 hours at a temperature of 430 C. (wall temperature 440450 C.). A maximum pressure of 825 atmospheres was reached. The reaction product, which weighed 67.6 gm., was worked up in the manner described in the preceding examples. 1.9 gm. terephthalic acid were obtained.

Example VI 27.6 gm. potassium carbonate, 5.0 gm. cadmium fluoride, 25.6 gm. calcium carbide and 40 gm. asbestos were milled and intimately admixed with each other in a ball mill. The resulting mixture was placed into an autoclave having a net volume of 600 cc. Thereafter, 250 cc. dry benzene were added to the mixture and 50 atmospheres carbon dioxide were introduced into the autoclave until saturation. The contents of the autoclave were then heated for 6 hours at a temperature of 430 C. (wall temperature 430-450 C.). The maximum pressure reached during that period was 780 atmospheres. The raw reaction product, which weighed 107.8 gm., was admixed with 50 gm. potassium carbonate and the mixture was then added to 600 cc. water and the solution was heated to the boiling point for half an hour. The solution wasqthen worked up as previously. described. The yield of terephthalic acid was 0.8 gm. I

Example VII 1 27.6 gm. potassium carbonate, 5.0 gm.- cadmium fluoride, 10.0 gm. aluminum carbide and 20 gm. asbestos were milled and intimately admixed with each other in a ball mill. ,Thereafter, the resulting mixture Was stirred into 96 gm. naphthalene and the mixture was placed into an autoclave having a net volume of 600 cc. Thereafter, the contents of the autoclave were heated for 12 hours at a temperature of 420 C. (internal temperature). At the beginning of the run and prior to heating, 50 atmospheres carbon dioxide were introduced into the autoclave. During the heating phase, additional amounts of carbon dioxide were introduced with the aid of a compressor until the internal pressure at 420 C. reached 1350 atmospheres. The reaction product was boiled with about 500 cc. water and the resulting solution was allowed to cool, and was then filtered. The aqueous filtrate was worked up in the above-described manner. 2.1 gm. naphthalene-2,6-dicarboxylic acid were obtained. By extraction of the mother liquor with ether, 0.4 gm. of a naphthalene-polycarboxylic acid mixture was obtained.

Example VIII A mixture of 27.6 gm. anhydrous potassium carbonate, 10.0 gm. aluminum carbide and 0.6 gm. cadmium fluoride was milled in a ball mill and thereafter placed into an autoclave having a net volume of 500 cc. Subsequently, 400 cc. benzene were added thereto and then carbon dioxide was introduced into the autoclave at a temperature of 40 C., until the internal pressure reached 50 atmospheres. The contents of the autoclave were then heated for 40 hours at 410 C. Upon heating the autoclave, the pressure increased sharply and was maintained at about 1700 atmospheres by manipulation of the exhaust valve. The raw reaction product, which weighed 49.05 gm., was worked up in the manner described above. By crystallization of the acidified aqueous solution and subsequent extraction of the mother liquor with ether, a totalof 7.2 gm. trirnesic acid was obtained.

Exampl IX 32.6 gm. cesium carbonate, 10.0 gm. aluminum carbide, 3.0 gm. cadmium fluoride and 20.0 gm. asbestos were intimately admixed with each other by milling in a ball mill, and the resulting mixture was heated in an autoclave having a net volume of 250 cc., together with 130 cc. benzene, for 2 hours at 380 C., and then for 14 additional hours at 400 C. Prior to heating (at 40 C.) carbon dioxide was introduced until the internal pressure reached 50 atmospheres; during the heating period the superatmospheric pressure developed thereby was maintained at about 1500 atmospheres by manipulation of the exhaust valve. Upon cooling and releasing the pressure from the autoclave, the reaction product, which weighed 73.15 gm., was worked up in the manner described above. Upon acidifying the aqueous solution with hydrochloric acid, 1.7 gm. terephthalic acid were obtained, and by subsequent extraction of the mother liquid with ether 2.35 gm. benzene polycarboxylic acids (acid No.=812) were obtained.

Example X 27.6 gm. anhydrous potassium carbonate, 10.0 gm. aluminum carbide, 60 gm. mercuric chloride and 20.0 gm. asbestos were intimately admixed by milling in a ball mill, and the resulting mixture was heated in an autoclave having a net volume of 600 cc., together with 300 cc. benzene, for 12 hours at 400 C. Prior to heating (at 40 C.) carbon dioxide was introduced into the autoclave until the internal pressure reached 50 atmospheres. The internal pressure at 400 C. was about 760 atmospheres.

answer E Thereaction product was worked up in the manner above described and yielded 0.7 gm. trirnesic acid;

Example XI 27.6 gm. anhydrous potassium carbonate, 10.0 gm. aluminum carbide, 6.0 gm. powdered cadmium and 20.0 gm. asbestos were intimately admixed by milling in a ball mill, and the resulting mixture was heated in an autoclave having a 'net volume of 600 cc., together with 300 cc. ben; zene, for 12 hours at 400. C. .Prior to heating '(at,40 C.) carbon dioxide was introduced into the autoclave until the internal pressure reached 50 atmospheres. At 400 C. an internal pressure 0f700 atmospheres developed. The reaction product, which weighed 68.8 gm., was worked up in the manner described above. 1.9 gm. terephthalic acid and 2.25 gm. trirnesic acid were obtained.

Example XII 27.6 gm. anhydrous potassium carbonate, 12.0 gm. aluminum phosphide, 10.0 gm. cadmium fluoride and 20.0 gm. asbestos were intimately admixed by milling in a ball mill, and the resulting mixture was heated in an autoclave having a net volume of 600 cc., together with 300 cc. benzene, for 12 hours at 420 C. Prior to heating (at 40 C.) carbon dioxide was introduced into the autoclave until the internal pressure reached 50 atmospheres. At 420 C. the internal pressure reached 910 atmospheres. The reaction product, which weighed 75.4 gm., was Worked up in the manner described above. 0.9 gm. terephthalic acid as well as 0.35 gm. trirnesic acid were obtained.

Example XIII 27.6 gm. anhydrous potassium carbonate, 10.0 gm. alu minum carbide, 6.0 gm. cadmium fluoride and 20.0 gm. asbestos were intimately admixed by milling in a ball mill, and the resulting mixture was heated in an autoclave having a net volume of 600 cc., together with 50 cc. benzene, for 18 hours at 420 C. Prior to heating (at 40 C.) carbon dioxide was introduced into the autoclave until the internal pressure reached 50 atmospheres. At 420 C. the internal pressure reached 260 atmospheres. The reaction product, which weighed 75.3 gm., was worked up in the manner described above. 1.7 gm. terephthalic acid as well as 2.5 gm. trirnesic acid were obtained.

Example XIV sure increased sharply. By a suitable manipulation of the v exhaust valve, the internal pressure was maintained at about 1700 atmospheres during the entire reaction period. The reaction product, which weighed 79.5 gm., was worked up in the manner above described. 20.7 gm. terephthalic acid as well as 0.45 gm. trirnesic acid were obtained.

Example XV 32.6 gm. cesium carbonate, 3.0 gm. cadmium fluoride and 10.0 gm. aluminum carbide (particle size 0.06 mm.) were intimately admixed by milling in a ball mill, and the resulting mixture was heated in an autoclave having a net volume of 250 cc., together with cc. benzene, for 20 hours at 380 C. Prior to heating, carbon dioxide was introduced into the autoclave until the internal pressure reached 50 atmospheres. The maximum pressure at 380 C. was about 1500 atmospheres. The reac tion product, which weighed 54.2 gm., was worked up in the manner described above. 7.0 gm. terephthalic acid were obtained, which corresponds to a yield of 42.2% of theory, based on the amount of cesium carbonate originally used. By extraction of the mother liquor with ether, 0.75 gm. tn'mesic acid were obtained.

Example XVI 27.6 gm. anhydrous potassium carbonate, 6.0 gm. cadmium fluoride and, as a water-binding agent, elemental silicon (crystallized, graphite form) were intimely admixed by milling in a ball mill. The resulting mixture was heated in an autoclave having a net volume of 600 cc., together with 325 cc. benzene, for 16 hours at 410 C. Prior to heating, a suflicient amount of carbon dioxide was introduced into the autoclave from a steel cylinder to produce a total pressure of 1470 atmospheres at the reaction temperature. The reaction product, which weighed 57.45 gm., was dissolved in hot water. The solution was filtered and the filtrate was heated to the boiling point and then acidified with hydrochloric acid. 3.25 gm. terephthalic acid were obtained and extraction of the mother liquor with ether yielded 0.35 gm. trimesic acid.

Example XVII 27.6 gm. potassium carbonate, 6.0 gm. cadmium fluoride and 22.4 gm. silicon (commercial designation: technical, powdered) were milled in a ball mill. The resulting mixture was heated in an autoclave having a net volume of 600 cc., together with 300 cc. benzene, for 16 hours at 410 C. Prior to heating, a sufficient amount of carbon dioxide was introduced into the autoclave to produce a total pressure of 1550 atmospheres at the reaction temperature. The reaction product, which weighed 72.35 gm., was worked up in the abovedescribed manner and yielded 4.1 gm. terephthalic acid. By extraction of the mother liquor with ether, 0.6 gm. trimesic acid were recovered.

Example XVIII 27.6 gm. potassium carbonate, 6.0 gm. cadmium fluoride and, as a water-binding agent, 21.6 gm. boron (amorphous, powdered) were milled in a ball mill. The resulting mixture was heated in an autoclave having a net volume of 600 cc., together with 325 cc. benzene, for 16 hours at 410 C. Prior to heating, a suflicient amount of carbon dioxide was introduced into the autoclave to produce a total pressure of 1525 atmospheres at the reaction temperature. The reaction product, which weighed 57.25 gm., was worked up in the manner described above and yielded 1.7 gm. terephthalic acid.

Example XIX 13.8 gm. potassium carbonate, 0.5 gm. cadmium fluoride and, as water-binding agent, 25.5 gm. tetraphenylsilicon were milled in a ball mill. The resulting mixture was heated into an autoclave having a net volume of 600 cc., together with 300 cc. benzene, for 17 hours at 410 C., and then for 8 additional hours at 420 C. Prior to heating, a sufiicient amount of carbon dioxide was introduced into the autoclave to produce a total pressure of 1340 atmospheres at the reaction temperature. After decanting the benzene, which contained a portion of the unreacted tetraphenyl-silicon, the reaction product was again washed with boiling benzene and then worked up in the above-described manner. 5.1 gm. terephthalic acid were obtained thereby.

The above examples disclose various features of the invention. For example, Examples IX, XIV and XV show the effect of other acid-binding agents-than the preferred potassium carbonate. Examples VI, XII and XVI to XIX show theetfect of various water binding agents as compared to aluminum carbide. Examples IV and XIII show the efi'ect of a reductionin the large excess of benzene which causes a reduction in the operating pressures. Example VII shows the use of other aromatic hydrocarbons. Examples III and VIII show the efiect of eliminating the inert solid diluent. Examples II, V, X and XI show the use of varying catalysts and Example VIII shows the effect of varying the amount of catalyst and how it will affect the polycarboxylic acid produced by the reaction.

While we have described particular embodiments of our invention, it will be understood that the invention is not limited thereto and that various modifications and adaptations thereof may be made without departing from the scope of the invention as set forth in the above disclosure and the following claims.

We claim:

1. A process for the introduction of carboxyl groups into aromatic hydrocarbons selected from the group consisting of benzene and naphthalene which comprises heating said aromatic hydrocarbon to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place in the presence of carbon dioxide, an alkali metal carbonate as an acid binding agent and a water binding compound selected from the group consisting of aluminum carbide, nitride and phosphicle, alkali and alkaline earth metal carbides, nitrides and phosphides, boron, silicon, and tetraphenyl silicon, and recovering carboxylic acids of said aromatic hydrocarbons having from two to four carboxyl groups in the form of their salts.

2. The process of claim 1 wherein the reaction is carried out under superatmospheric pressure in the presence of a catalyst containing a metal selected from the group consisting of zinc, cadmium, mercury, iron and lead.

3. The process of claim 1 wherein the reaction is carried out in the presence of a solid inert diluent.

4. The process of claim 1 wherein said alkali metal carbonate acid binding agent is potassium carbonate and said water-binding compound is aluminum carbide.

5. A process for the production of terephthalic acid which comprises heating benzene to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place under superatmospheric pressure in the presence of carbon dioxide, an alkali metal carbonate as an acid binding agent, a water-binding compound selected from the group consisting of aluminum carbide, nitride and phosphide, alkali and alkaline earth metal carbides, nitrides and phosphides, boron, silicon, and tetraphenyl silicon, and a catalyst containing a metal selected from the group consisting of zinc, cadmium, mercury, iron and lead and recovering terephthalic acid in the form of its salt with said alkali metal.

6. A process for the production of terephthalic acid which comprises heating benzene to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place, under superatmospheric pressure in the presence of excess carbon dioxide, potassium carbonate as an acid-binding agent, aluminum carbide as a water-binding agent, and a cadmium-containing catalyst and recovering terephthalic acid in the form of its potassium salt.

7. A process for the production of naphthalene-2,6- dicarboxylic acid which comprises heating naphthalene to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place under superatmospheric pressure in the presence of carbon dioxide, an alkali metal carbonate as an acid binding agent, a water-binding compound selected from the group consisting of aluminum carbide, nitride and phosphide, alkali and alkaline earth metal carbides, nitrides and phosphides, boron, silicon, and tctraphenyl silicon, and a catalyst containing a metal selected from the group consisting of zinc, cadmium, mercury, iron and lead and recovering naphthalene-2,6-dicarboxy1ic acid in the form of its salt with said alkali metal.

8. A process for the production of naphthalene-2,6- dicarboxylic acid which comprises heating naphthalene to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place under superatmospheric pressure in the presence of excess carbon dioxide, potassium carbonate as an acid binding agent, aluminum carbide as a water binding agent, and a cadmium-containing catalyst, and recovering naphthalene- 2,6-dicarboxylic acid in the form of its potassium salt.

9. A process for the production of trimesic acid which comprises heating benzene to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place under superatmospheric pressure in the presence of carbon dioxide and alkali metal carbonate as an acid binding agent, and a water-"binding compound selected from the group consisting of aluminum carbide, nitride and phosphide, alkali and alkaline earth metal carbides, nitrides and phosphides, boron, silicon, and tetraphenyl silicon, and a catalyst containing a metal selected from the group consisting of zinc, cadmium, mercury, iron and lead and recovering trimesic acid in the form of its salt with said alkali metal.

10. A process for the production of trimesic acid which comprises heating benzene to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place under superatmospheric pressure in the presence of excess carbon dioxide, potassium carbonate as an acid binding agent, aluminum carbide as a waterbinding agent, and a cadmium-containing catalyst and recovering trimesic acid in the form of its potassium salt.

11. A process for the introduction of can-boxy-l groups into aromatic hydrocarbons selected from the group consisting of benzene and naphthalene which comprises heating said aromatic hydrocarbon to a temperature above about 350 C. and below the temperature at which substantial decomposition of the starting material and reaction products takes place in the presence of carbon dioxide and an alkali metal carbonate in excess over that amount required as an acid-binding agent for the neutralization of the carboxylic acid formed by the reaction as a water-binding compound, and recovering carboxylic acids of said aromatic hydrocarbons having from 2 to 4 carboxyl groups in the form of their salts.

12. The process of claim l l wherein said alkali metal carbonate is potassium carbonate.

References Cited in the file of this patent UNITED STATES PATENTS 150,867 Kolbe May 12, 1874 1,866,7 l7 Meyer et a1. July 12, 1932 1,937,477 Mills et a1 Nov. 28, 1933 2,685,600 Morris et a1. Aug. 3, 1954 2,794,830 Raecke et al. June 4, 1957 2,823,231 Raecke et a1 Feb. 11, 1958 UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent N0o 2,948 75O August 9, 1960 Bruno Blaser et a1 It is hereby certifiedthat error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 5 line 32., for "500 coo" read mm 600 co. line 68, for "60 gm,.," read 6.0 gm.

Signed and sealed this 18th day of July 1961,

(SEAL) Attest:

ERNEST W. SWIDER DAVID L. LADD Attesting Officer Commissioner of Patents 

1. A PROCESS FOR INTRODUCTION OF CARBOXYL GROUPS INTO AROMATIC HYDROCARBONS SELECTED FROM THE GROUP CONSISTING OF BENZENE AND NAPHTALENE WHICH COMPRISES HEATING SAID AROMATIC HYDROCARBON TO A TEMPERATURE ABOVE ABOUT 350*C. AND BELOW THE TEMPERATURE AT WHICH SUBSTANTIAL DECOMPOSITION OF THE STARTING MATERIAL AND REACTION PRODUCTS TAKES PLACE IN THE PREXENCE OF CARBON DIOXIDE, AN ALKALI METAL CARBONATE AS AN ACID BINDING AGENT AND A WATER BINDING COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALUMINUM CARBIDE, NITRIDE AND PHOSPHIDE, ALKALI AND ALKALINE EARTH MEAL CARBIDES, NITRIDES AND PHOSPHIDES, BORON, SILICON, AND TETRAPHENYL SILICON, AND RECOVERING CARBOXYLIC ACIDS OF SAID AROMATIC HYDROCARBONS HAVING FROM TWO TO FOUR CARBOXYL GROUPS IN THE FORM OF THEIR SALTS. 